Recent communication systems have attempted to expand protocols by adding new lower layer protocols while retaining higher layer communication protocols by, for example, designing wireless media access control/physical (MAC/PHY) which is originally for a wired communication, as utilized in a wireless universal serial bus (USB) or wireless 1394, and standardizing alternative MAC/PHY for support of high speed and new radio frequency (RF) bands, as defined in IEEE 802.11(WLAN) and IEEE 802.15(WPAN).
The communication system is not configured by only one protocol, but is evolving by utilizing multiple higher layer protocols and multiple lower layer protocols to constitute one system which chooses a protocol suitable for use in a situation.
Currently used protocols are designed to use own specific base clock, resulting in one system having multiple clock sources. Because most wireless communication protocols require synchronization of clocks used in a lower physical layer for communication, clock synchronization of physical layers has been defined, but a method for synchronization of clocks of higher layers has not been yet defined.
Some applications have no problems in communication if clocks of lower physical layers are synchronized, but other specific applications should perform multi-clock synchronization for higher layer protocols for communication.
FIG. 1 illustrates a diagram of an example of a conventional high definition multimedia interface (HDMI) transmitter, and FIG. 2 illustrates a diagram of an example of a conventional HDMI receiver.
Referring to FIGS. 1 and 2, the HDMI transmitter and the HDMI receiver employ HDMI protocol adaption layer (PAL) specified in ECMA-387, which is standard for wireless high-speed transmission of large multimedia data. The HDMI transmitter and the HDMI receiver utilize HDMI protocol using mm-wave of 60 GHz band to transmit data wirelessly.
In transmission of multimedia data such as video and audio data, if an HDMI source shown in FIG. 1 and an HDMI sink clock of the example illustrated in FIG. 2 are not synchronized with each other, video buffer overflow or underflow occurs in video buffer of a receiving terminal due to clock errors accumulated for a certain period of time, causing damage to data.
In ECMA-387 standard, clock synchronization in units of packets is defined for physical (PHY) layer of each of a transmitter and a receiver in the 60-GHz wireless communication band, and synchronization in units of given periods of time, i.e., superframes, is defined for MAC layer. Furthermore, ECMA-378 standard defines a clock synchronization method for PAL layer.
As a synchronization method for a lower layer such as MAC/PHY layer, an existing stable synchronization method used for other protocols is employed to prevent errors in the synchronization of the lower layer, which may cause communication failure. On the other hand, for clock synchronization of HDMI PALs of a source terminal and a sink terminal, under the assumption that a delay time is inserted between the time of generation of a packet and the time of reception, data to be transmitted from a transmitting terminal PAL to a receiving terminal PAL includes time information about the packet generation time, and the receiving terminal PAL compares the time information included in the received data with a packet reception time, thereby synchronizing a clock of the receiving terminal to a clock of the transmitting terminal.
However, because clock information storage time differs in the transmitting terminal and the receiving terminal and different delay times are inserted into the packet at each of different lower layers while the packet has passed through the lower layers, synchronization errors grow over time, causing an unstable clock.
In an effort to correct the accumulative errors, a large amount of data is devoted to clock information, and it reduces the data efficiency. In addition, an error that has once occurred is accumulated, resulting in system malfunction or failure.
As such, since different protocols use different clock sources or different synchronization methods and errors in synchronization between the transmitting terminal and the receiving terminal lead to a serious system problem in a multi-clock system in which lower layers and higher layers use heterogeneous clocks, a clock synchronization method is required which is capable of stably maintaining synchronization even when variable delay times are inserted into a packet while passing through different lower layers.